The invention relates to an arrangement for determining the extreme values from the density values in a plurality of individually scanned small scanned points of predetermined size in a certain section of a photographic original forming part of a strip of such originals. More specifically, the invention relates to the type of scanning arrangement which, for example, includes an objective projecting the image of the original, a rotating apertured disk located in an intermediate plane such that the apertures move through a stationary beam of light passing through the strip, a photoelectric transducer positioned behind the apertured disk to receive the light from a scanning spot formed by the apertured disk and generating a corresponding density-indicating electrical signal, and an evaluating arrangement for determining the extreme values of the electrical signal.
Federal Republic of Germany Pat. No. 1,070,492, and corresponding U.S. Pat. No. 3,034,400, disclose an exposure control arrangement provided with means for measuring the range of density values of each original to be printed. This is accomplished by using a semitransmissive mirror to extract from the main exposure light beam a supplemental beam which is passed through a Nipkow disk arranged in front of a photoelectric transducer. In this way, all points on the original to be copied are scanned one after the other and the density of such points measured by the photoelectric transducer. A complex electric circuit receives the signal from the photoelectric transducer and determines the minimum and maximum densities of the original to be copied, and also the range of density values of the original.
With this known arrangement the scanning is preformed while the original to be printed, and accordingly the strip of which the original forms one unit, is at a standstill. The scanning spot must be moved over the entire original, or the fraction thereof to be scanned, travelling both from side to side in the traversal of one scan line and top to bottom in moving from one scan line to the next. This scanning is performed either before or during printing of the original, with the measuring light beam for the scanning spot in part coinciding with the copying light beam for the printing operation. If the scanning is performed during the actual printing exposure, then the scanning must be completed before the end of the exposure time, and the exposure time may be very short for certain originals to be copied. As a result, if it should happen that the proper or correct exposure time has a certain minimum value, then the exposure will be undesirably prolonged because the scanning will continue until the completion of the scanning cycle to yield the density-range information ordinarily necessary to determine the duration (and/or intensity) of the printing exposure. The use of a separate photoelectric scanning means which does not derive its scanning light from the exposure light beam, in conjunction with an intermittent transport of the originals to be copied, gives rise to the problem that in the scanning station each original must be brought to a stop at a position corresponding very exactly to the position in the copying station.